1. Field of the Invention
The present invention relates to a voltage subtracting circuit, an intensity detecting circuit, and a semiconductor integrated circuit device. The present invention relates to a technique such as Bluetooth which is used for radio communications.
2. Description of the Related Art
In recent years, much attention has been paid to Bluetooth, which is a short distance wireless communication system that connects mobile apparatuses such as notebook personal computers, PDAs (Personal Digital Assistants), and cellular phones together.
With a radio communication system such as Bluetooth, the intensity of electric waves varies significantly depending on the distance between a transmitting apparatus and a receiving apparatus. Accordingly, the receiver must have a mechanism that adjusts an amplification factor depending on the intensity of a received signal to stabilize the signal intensity. Such a system has been proposed in, for example, Hiroki Ishikuro et al., “A Single-Chip CMOS Bluetooth Transceiver with 1.5 MHz IF and Direct Modulation Transmitter”, ISSCC Digest of Technical Papers, February 2003, p. 94 to 95 and Katsuji Kimura, “A CMOS Logarithmic IF Amplifier with Unbalanced Source-Coupled Pairs”, IEEE Journal of Solid-State Circuits, Vol. 28, No. 1, January 1993, p. 78 to 83. However, the detection characteristic of the conventional mechanism is dependent on parameters for circuits and devices. Thus, with the conventional mechanism, stable detections are difficult.